In recent years, liquid crystal displays have been rapidly developed and spread. This trend caused an increase in demand for cellulose acylate films, especially triacetyl cellulose (TAC) films that are used as a protective film for the liquid crystal displays. Along with the increase in demand, the productivity of the TAC films is desired to be improved. The TAC films are often produced according to the following method. At first, a dope including TAC and a solvent is cast using a casting die onto a continuously running support to form a casting film. The casting film is dried or cooled to develop its self-supporting properties. After having possessed the self-supporting properties, the casting film is peeled from the support, and then dried and wound as a TAC film. According to such solution casting method, the produced film has few foreign substances mixed therein and has excellent optical properties, as compared to films produced according to a melt extrusion method.
In the solution casting method, the support for the dope may be a belt or a drum. To improve the casting speed, the drum is more preferred as compared to the belt. The casting film on the support is dried, or cooled and gelated in order to develop the self-supporting properties. Meanwhile, the film is stretched in order to control the optical properties, especially the retardation characteristics, of the produced TAC film.
Optimum speed of the film production and that of the film stretching vary from each other. The film production speed, that is, the casting speed is generally slower than the film stretching speed. Therefore, when the stretching speed is adjusted to the casting speed, the film may not be sufficiently stretched to improve the optical properties of the produced TAC film. In view of this, it is proposed that the film is stretched off-line, that is, outside a solution casting line (see, for example, Japanese Patent Laid-open Publication No. 2002-311240).
As disclosed in the Japanese Patent Laid-open Publication No. 2002-311240, when the film is stretched off-line, that is, independently from the solution casting line, it is preferable to stretch the film continuously so as to perform the film stretching effectively. In order to stretch the film continuously, a rear end of a leading film and a front end of a following film are connected using a connection tape.
During the stretching, both side edges of the film are held with holding members like clips. The film edges held with clips cannot be used as a product, and therefore they are slit to be separated from a main part of the film. The main part is going to be the end product. It is preferable to shred the slit edges into chips to be recycled for preparing the dope, in view of economic advantages.
However, the connection tape cannot be recycled for preparing the dope. Therefore, when the connection tape is used to connect the films, the connection tape needs to be removed from the films before or after the edges are slit. When the connection tape is removed, the slit edge of the leading film and the slit edge of the following film fall apart, which lowers the workability in recovering the slit edges. In addition, when the connection tape is not removed completely, the tape may be a foreign substance mixed into the recovered slit edges, which lowers the quality of the recycled TAC film. Such problems are not limited to the TAC film production, but may arise in any polymer film production where the slit film edges are recycled for preparing the dope.